Cytomegalovirus (CMV) is the most common opportunistic infection in solid organ transplant recipients, particularly in lung transplant recipients (LTRs). Our published data show that donor positive/recipient negative LTRs (D+R-) at high risk for CMV frequently develop CMV-specific immune responses, often with a persistence of CMV-specific effector memory T cells in the lung allograft and blood long after primary infection. Using our novel system to identify CMV-specific T cells in bronchoalveolar lavage-obtained allograft cells (allograft BAL cells) and peripheral blood mononuclear cells (PBMC), this proposal will extend our previous work and examine more closely the CMV memory pools in these distinct tissue compartments. Our published observation that CMV-specific CD4+ T cells can be differentially detected in the BAL cells when undetectable in the PBMC, has led us to hypothesize that there are differences in the frequency, phenotype and function of CMV memory cells in the allograft compared to PBMC. This will be explored in SA1 using a variety of CMV antigens, CMV class I tetramers and multi-parameter flow cytometric analysis. Because the majority of D+R- LTRs develop active CMV infection, we will test our hypothesis that CMV-specific cellular responses change during the transition from active primary infection to latent infection in the lung allograft and PBMC in SA2. Our exciting preliminary data show robust de novo CMV-specific T cell responses in both tissue compartments that we have prospectively captured in 6 D+R- LTRs. It has been postulated that the MHC-mismatch barrier may limit CMV host defense, particularly at the level of the allograft. Using cryogenically preserved donor splenocytes or PBMC we will test the ability of LTRs to exhibit CMV-specific effector responses in the context of donor antigen presenting cells (APC) compared to autologous APC in SA3. The PI, John McDyer MD, a K08 awardee and Assistant Professor, is a transplant pulmonologist in the Johns Hopkins Division of Pulmonary/Critical Care Medicine and an NIAID-trained immunologist committed to advancing our understanding in the field of transplant immunobiology and transplant-related host defense. This R21 award will provide the foundation for future patient-based translational studies examining the dynamics between CMV, the allograft, and host immune system that may increase our knowledge of CMV-specific T cell memory and its potential clinical implications in transplant. Cytomegalovirus (CMV) is the most common infection in solid organ transplant recipients, particularly lung transplant recipients. CMV infection is associated with increased risk for both acute and chronic rejection in lung transplant recipients, though it is unclear why. Understanding the immune response to CMV infection in susceptible lung transplant recipients may improve our treatment strategies and ultimately impact long-term outcomes in transplant recipients. [unreadable] [unreadable] [unreadable]